The present invention relates to a non-linear optical effect frequency conversion laser, and more particularly, to a non-linear optical effect frequency conversion laser which uses a ring resonator.
There has been known technology which uses a non-linear optical effect by allowing the laser light to enter a non-linear medium to generate higher harmonics. For instance, the light of the frequency of 2.omega. can be achieved simply by making the laser light having the frequency of .omega. to enter a second-order non-linear medium suitable for the generation of second harmonics (SHG) which is not symmetrical about the center thereof. Therefore, the non-linear medium is frequently used as a frequency conversion element for converting a frequency to a higher one (or a wavelength to a shorter one). A requirement for a high-efficiency SHG is phase matching that is achieved when the non-linear medium exhibits birefringence, and furthermore, crystal orientation and polarization layout by which the refractive index of a second harmonic is coincident with that of a fundamental harmonic are existent in the medium. Examples of the second-order non-linear material (SHG element) capable of phase matching include KH.sub.2 PO.sub.4 (KDP), LiNbO.sub.3, KNbO.sub.3, KTP, Ag.sub.3 AsS.sub.3, and CO(NH.sub.2).sub.2 (UREA). However, when light having a predetermined frequency is taken out simply by causing the laser light to enter the SHG element for second harmonic generation, output efficiency is low.
To overcome this low output efficiency, non-linear optical frequency conversion technology which uses a monolithic ring resonator made of KNbO.sub.3 as a second-order non-linear material to generate second harmonics has been proposed. The bulletin Vol.56 No.23 of the American Applied Physics Society published on Jun. 4, 1990 discloses on p.2291-2292 a second order optical frequency convertor which comprises a diode laser 11, a lens 13, a pair of prisms 14, an isolator 15, a lens 17, an SHG element 19, a photo detector 21 for detecting light reflected from the SHG element 19, an RF (radio frequency) amplifier 23 for amplifying a detection signal from the photo detector 21, a double-balanced mixer 25, an RF (radio frequency) signal generator 27 for generating an RF (radio frequency) signal to input the RF signal into the double-balanced mixer 25 and to mix the control signal of the diode laser 11 and the RF signal through a filter 29, an amplifier 32 for amplifying a heterodyne signal from the double-balanced mixer 25 for feedback output to the diode laser 11, and a DC (direct current) power supply 33 for adding a DC bias element to the output of the amplifier.
In this device, laser light having a wavelength of 856 nm, for example, from a GaAlAs diode laser is inputted into one end of the monolithic ring resonator made of KNbO.sub.3 to output laser light having a wavelength of 428 nm, namely blue laser light, from the other end of this resonator. For such non-linear optical frequency conversion, the oscillation frequency of the diode laser needs to substantially accord with the resonance frequency of the ring resonator. In this example, the frequency of the diode laser fluctuates up to 2 GHz/mA by current variations and up to 40 GHz/.degree. C. by temperature variations. The temperature variations are caused by thermal variations due to: (1) changes in ambient temperature, and (2) a self-heating phenomenon optically induced by the diode laser itself. Therefore, this leads to variations in the output of infrared light from the diode laser, which affect the output of blue laser light outputted from the ring resonator as a second harmonic.
Therefore, so as to generate a second harmonic from a ring resonator stably, this embodiment discloses as an essential element of this non-linear optical frequency convertor, means for keeping the oscillation frequency of a diode laser constant in the following steps: (1) superimposing a reference low frequency on the sideband of the laser oscillation frequency, (2) reflecting part of the laser light containing the low frequency component with one end of the resonator, (3) detecting the reflected light with the photo detector, (4) detecting the state of resonance of the ring resonator from the signal of the reflected light, and (5) feed-back controlling of the oscillation of the diode laser based on the detected state of resonance.
However, in the conventional non-linear optical effect frequency conversion laser, frequencies are not stable in terms of long intervals. In spite of the temperature control of the non-linear optical crystals, the semiconductor laser generates heat after long-time use, thus changing crystal composition and oscillation frequency. In the conventional device, output fluctuates by the control of frequency because laser output and oscillation frequency are controlled simultaneously only with a current.